1. Field of the Invention
The present invention relates to star-shaped nylons, methods for their preparation, tetrasubstituted carboxylic acids and methods for their preparation. More particularly, it is concerned with star-shaped nylons with desired properties provided by use of characteristic polymerization cores, new tetrasubstituted carboxylic acids for use as the aforementioned polymerization cores and methods for their preparation.
2. Description of the Related Art
The so-called star-shaped polymers are macro molecules of a structure where a plurality of radial polymer chains emanate from a polymerization core as a center, their chemical structure being characterized in that, as compared with the conventional linear polymers, the molecular weight per polymer chain is less, and the respective polymer chains combine with each other through the polymerization core. Therefore, star-shaped polymers have relatively less entanglement of respective polymer chains, which contributes to their relatively low melt viscosities. This often leads to their preferable physical properties to enable injection molding into films, or leads to excellent compatibility with other polymers to broaden the possibilities of polymer blends.
Also relating to nylons, those of star-shaped type have been investigated as exemplified in U.S. Pat. No. 4,599,400 and "Polymer Preprints," 30(1), pp117-118, American Chemical Society, 1989. For example, a star-shaped nylon 6 is described therein which is prepared by making use of a star-shaped amine compound with plural amino groups on separate positions in a molecule as a polymerization core, and subjecting the respective amino groups to ring-opening polymerization with .epsilon.-caprolactam or a nylon monomer.
The above-mentioned type of star-shaped nylon, however, does not always exhibit low melt viscosity, nor good mechanical properties (e.g. tensile strength, tensile modulus, etc.). These drawbacks are assumed to be due to the following causes characteristic of nylons.
That is, polymer chains of nylon contain numerous amide bond (--CO--NH--) portions which are indispensable to crystallization due to formation of hydrogen bonds between nylon molecules upon solidification, and eventually contribute to improvement in various mechanical properties of nylon materials.
In the early stage of polymerization for star-shaped nylons, however, the respective polymer chains in the same molecule are present in close vicinity to each other near the polymerization core, and further the reactivity of the amide bond portions of the respective polymer chains remain high until completion of polymerization. So the mutual contact of the respective polymer chains in the same molecule causes radical formation due to deprotonation at the amide bond portions and thus formation of intramolecular network structure among the polymer chains. Such intramolecular network structure is known to cause an increase in melt viscosity of nylon.
The intramolecular network structure formed between the amide bond portions prevents formation of hydrogen bonds among nylon molecules caused by amide bonds upon the solidification, resulting in poor crystallization which provides nylon materials with inferior mechanical properties.
For the foregoing reasons, it is necessary to keep the respective polymer chains in the molecule from their mutual close contact in the early stage of polymerization in order to prepare star-shaped nylons having low melt viscosities and excellent mechanical properties.
According to the prior art, however, a star-shaped amine compound as a polymerization core is not of a rigid molecular structure, and eventually infallible separation of the respective polymer chains from each other cannot be attained in the course of polymerization, thereby failing to prevent formation of the intramolecular network structure discussed above, even if a plurality of amino groups are positioned separated from each other in the molecule so that the respective polymer chains in the molecule do not mutually contact.